As shown in U.S. Pat. No. 6,761,658 infinitely variable transmissions (IVT) are known in the art to utilize a hydro-mechanical transmission with a two row planetary gear set that gives four forward travel modes, and a reversing planetary that gives reverse travel. Four torque-coupling devices are used. The first can be in a neutral position, position “C1,” or position “C2.” It must be in C1 or C2 when torque is desired. In addition, one of the remaining torque-coupling devices must be engaged for final torque delivery to occur (one would connect the carrier to the output shaft, one would connect the sun gear in row 2 to the output shaft, and one would be used to enable the reversing planetary which connects the reverse carrier speed to the output shaft). Two hydrostatic units are used in a closed hydraulic loop, both of which are variable displacement. The first hydrostatic unit, “CU,” is connected to the torque path via C1 (hydrostatic unit coupled to carrier) or C2 (hydrostatic unit coupled to the sun gear in row 2), depending on the position of the synchronizer. The second hydrostatic unit, “RU,” is permanently connected to the ring gear of the planetary gear set. The hydrostatic unit's respective displacements per revolution vary with the angle between their swashplate and piston block, with zero degrees being zero displacement. Positive and negative angles are possible with both hydrostatic units, to give a broad speed range. Their angle is controlled by a servo system and two electro-hydraulic proportional valves. The hydrostatic units can act as pumps or motors, depending on the transmission mode. The planetary gear set sums torque provided directly from the engine with that provided by the hydrostatic units (indirect engine torque). The result is a high-efficiency infinitely variable ratio transmission with a speed range and torque capability suited to agricultural and industrial applications. The transmission speed can be measured with a speed sensor on the ring gear as disclosed in U.S. Pat. No. 6,485,387. A speed sensor is also used on CU, in order to compare with the transmission speed to ensure the first torque-coupling device is properly engaged.
The transmission of U.S. Pat. No. 6,761,658 uses a synchronizer to couple the CU hydro to C1 or C2. It is spring-biased to the neutral position, and pressure is applied to one side or the other to cause engagement. Also, for the synchronizer to engage properly there must be a relatively low torque, which for the most part means there is a small speed difference between the CU hydrostatic unit and the gear to be engaged (C1 or C2). The function of the synchronizer could also be implemented with a dog-clutch, a series of clutches, or similar means, but in any case it would be desirable to have minimal speed difference for engagement.
C1 is engaged at startup so that torque delivery can be achieved with CU coupled to the carrier, after gear is commanded. Before an output speed is desired, the transmission should be fully engaged with a wheel speed of zero. This is accomplished by allowing the CU hydrostatic unit to stay at its at-rest position, which gives a high displacement, and positioning the RU hydrostatic unit to be at zero degrees, which allows any RU/ring speed but prevents CU (and therefore C1, the carrier, and the output shaft once it is coupled to the carrier) from turning.
The current technology that positions RU at zero degrees and prepares the transmission to transmit torque to the wheels uses a sensor that reports the RU hydrostatic unit angle. Closed-loop control is used to swing the RU hydrostatic unit angle until it reaches zero (input is the hydrostatic unit angle from the sensor, output is electrical current to the valve that controls RU's position). In addition, when the RU angle is known, it can be used to position the hydro precisely during shifts, check for overspeed conditions (dependent on angle), etc. However, an angle sensor adds significant cost to the transmission and decreases overall reliability.